1. Field of the Invention
This invention relates to a method and apparatus for monitoring deterioration of a catalyst that is disposed inside an exhaust pipe for purifying the exhaust gas.
2. Description of the Related Art
To reduce detrimental components in an exhaust gas, it is known for the exhaust gas to be purified by a ternary catalyst, or the like, disposed inside an exhaust pipe of the internal combustion engine and such a catalyst is now a legal requirement in many States.
Such a catalyst undergoes gradual deterioration in the course of its use. If the catalyst degrades but is left unattended and the automobile is continuously driven, the exhaust gas is no longer naturally purified thereby causing social problems such as air pollution. In order to particularly prevent air pollution, it is a recent trend to also impose a legal obligation to detect deterioration of the catalyst and to give an alarm for deterioration of the catalyst to a driver, as typified by OBD-II, a regulation of the State of California, U.S.A.
Apparatus or methods for monitoring catalyst deterioration are described, for example, in Japanese Patent Laid-Open No. 97852/1988 and Japanese Patent Laid-Open No. 231252/1988. In both prior art documents an O.sub.2 sensor is provided at the input and output respectively of a catalyst converter. In operation, the output signal of the input sensor varies between high and low signal levels occasioned by so-called lambda O.sub.2 sensor feedback control for fuel injection. In the former prior art publication, the number of changes in signal at the input and output sensors are detected per unit of time and, if the catalyst is in good condition, the number of signal changes per unit of time at the output sensor should be much less than the changes at the input sensor. If the number of changes at the output sensor exceed a predetermined level per unit of time, deterioration of the catalyst is determined. In the latter document, the amplitude of the signal at the input and output sensors should have a predetermined difference (the output sensor amplitude being lower than that of the input sensor) and, if the difference drops below said predetermined level, deterioration of the catalyst is determined.
However, the apparatus and method used in the above prior art involves the problem that a relatively long period of time is necessary for performing the necessary calculation and, hence, delay in detecting catalyst deterioration occurs. Moreover, reliability of this detection result is not sufficiently high.
An object of the present invention is to provide a method and apparatus for determining catalyst deterioration speedily and accurately.